To detect, for example, the work reference position of a work set on the machining work, the position of a work (an object to be detected) set on the table is searched for by a needle mounted on a tool spindle. Then, the contact between the needle and the work is electrically detected and the work reference position of the work is calculated from the positions of the tool spindle and the table upon detection.
One of the known position detectors of such kind has a needle which contacts with an object to be detected to detach its base portion from a seat surface keeping the static position of the needle so that it closes the internal contact to produce a detecting signal. Other one has an external contact formed of electrically-insulated needle and the detected object so that the contact between the needle and the work closes the external contact to produce a detecting signal.
Further, a known position detector has a movable detecting head whose detecting signal is transmitted to a receiver and a controller located at the fixed position by means of radio waves.
In the apparatus in which the detecting signal is transmitted by radio wave, the receiver located at a predetermined position can receive the detecting signal without fail irrespective of the positional relationship between a detecting head which includes the detecting needle and the transmitter. Thus, this apparatus is free from a cumbersome wiring between the movable detecting head and the receiver and the controller located at the fixed position.
According to the position detector for transmitting a detecting signal by radio wave, particularly when the detecting head is mounted on the rotary tool spindle of a machining tool to detect the position of the work, the mounting structure of the antenna at a transmitter side causes a problem. Specifically, since this kind of apparatus employs a battery as the power source of the transmitter, it is strongly requested to produce an output as small as possible in order to avoid the battery from being considerably consumed The antenna, however, has a directivity, so that when the tool spindle is moved and rotated, the directivity of the antenna is changed and the radio wave is not reached to the receiver. Further, depending on the position at which the detecting head is located, the work and the main body of the receiver hide the antenna and the receiver, hindering the transmission of radio wave to the receiver. To avoid these problems, it is proposed to mount a plurality of antennas on the transmitter or the like. However, the mountable space thereof is restricted, and the mountable position thereof is also restricted so as to avoid the interference with the machining tool and the work when the detecting head is moved. For this reason, the effective technique for generally completely solving these problems has not been found out yet.
The first object of this invention is to provide a technique for solving the above problems fully and which can positively transmit a detecting signal by radio wave to a receiver located at a fixed position irrespective of the movement of the detecting head including the detecting needle and the transmitter.
The second object of this invention is to provide a position detector which can be prevented from being mis-operated when various extraneous noises are produced in the radio wave under the circumstances in which the position detector is operated.
The third object of this invention is to provide a position detector having advantages brought about by both the above position detector having the internal contact and the above position detector having the external contact Specifically, the position detector having the internal contact can detect the position of an object to be detected, independently of the property of the object to be detected, conductive or a non-conductive. This type of position detector, however, cannot actuate its internal contact without time delay after the detecting needle and the object to be detected are brought in contact with each other. There is then a strong possibility that various errors may occur. Thus, this internal contact type position detector produces measured values that are considerably made different as compared with the external contact type position detector. While, the position detector having the external contact can detect at high accuracy the position of the object made of conductive material but it cannot detect the position of the object made of non-conductive material Therefore, the third object of this invention is to solve these problems.